Alexandru Dobrovicescu

Refrigerated warehouses are among the most energy-intensive food storage facilities, with their thermal performance influenced by local climatic conditions and building envelope characteristics. This study investigates the influence of thermal behavior of a refrigerated warehouse, west wall, under three representative Romanian climatic conditions (Iasi, Cluj-Napoca, and Constanta), using dynamic simulations performed in TRNSYS and a steady-state calculation approach implemented in EES. The TRNSYS simulations were conducted using hourly meteorological data over a complete year (8760 h), while the evaluation focuses on hours 2089–7129 (5041 hourly observations), selected to examine the high cooling demand period. The analysis across the three locations examines the influence of outdoor air temperature and solar radiation on heat transfer through the refrigerated warehouse selected envelope and their influence on the refrigeration load required to maintain the prescribed indoor temperature. Relative humidity is considered separately in the assessment of condensation risk within the wall assembly. For Iasi, the transient thermal behavior of the west-oriented wall is examined in detail through the evolution of its indoor and outdoor surface temperatures and heat flux, together with a comparison between the TRNSYS dynamic simulation and the hourly quasi-steady-state calculation performed in EES. The findings indicate that local climatic conditions influence variations in heat transfer and refrigeration load during the investigated period, as expected, and emphasize the difference between the two approaches. For the wall examined in Iasi, the indoor surface temperature exhibits relatively small variations, whereas the heat flux does not always follow the instantaneous changes in outdoor air temperature. This behavior is consistent with heat storage and release within the multilayer wall assembly, reflecting the transient nature of heat transfer. The EES–TRNSYS comparison for Iasi allows differences between steady-state and dynamic representations of heat transfer to be identified, highlighting the relevance of the wall thermal response when interpreting heat flux variations under the investigated climatic conditions.

Liquid Air Energy Storage (LAES) is a promising large-scale storage technology for supporting the sustainable integration of intermittent renewable electricity into power grids, particularly when electricity storage is combined with waste-heat valorization, cryogenic cold recovery, and reduced exergy degradation. In this thermodynamic sense, the present study proposes a sustainability-oriented framework for the comparative and parametric exergetic analysis of LAES systems, integrating the liquefaction and discharge stages within a unified analysis. The assessment focuses on thermodynamic resource efficiency and exergy performance rather than on a complete economic, environmental, or life-cycle evaluation. The objective is to identify the components and operating parameters that most strongly influence performance and to quantify the reductions in exergy destruction and losses achieved through configuration changes and parameter variation. Three Linde–Hampson-based liquefaction configurations are compared, including arrangements with external and intermediate auxiliary pre-cooling. Improved heat-exchanger integration and temperature matching increase the structural liquefaction exergetic indicator from 7.95% in the baseline configuration to 19.28% in the two-RHX configuration. The discharge stage is assessed parametrically with respect to cryogenic pumping pressure, turbine inlet temperature, and expansion architecture. Single-stage and two-stage adiabatic expansions are compared with an ideal isothermal benchmark. The adiabatic configurations provide mechanical-work recovery together with recoverable cooling potential, whereas the isothermal case gives the highest work-recovery benchmark. Under the reference conditions, using the aggregated compressor representation adopted for the main parametric analysis, the two-stage adiabatic configuration reaches a global exergetic efficiency of 15.92% for the improved Linde–Hampson-based chain and 24.87% for the selected Claude–Heylandt reference block.

This study investigates steady-state conductive heat transfer and water-vapor diffusion through the external wall of a refrigerated warehouse with a specified load-bearing wall assembly. The formal analogy between heat conduction and mass diffusion is stated and used to establish a practical calculation framework for estimating heat and moisture ingress through multilayer cold-store walls. Calculation routines are presented to determine the temperature field and the corresponding water-vapor saturation and partial-pressure distributions across (and within) the insulation layer, enabling the identification of regions prone to interstitial condensation. The analysis highlights the roles of (i) the vapor diffusion resistance of the vapor barrier layer, (ii) the thermal resistance of the insulation, and (iii) key outdoor boundary conditions in governing condensation risk. Increasing insulation thermal resistance reduces external heat gains; however, it may also increase the likelihood of condensation in layers close to the cold side by lowering local temperatures and saturation pressures. Among external parameters, outdoor relative humidity exerts the strongest influence on interstitial condensation risk. For the investigated wall assembly, increasing outdoor relative humidity by 50% shifts the condensation onset location within the insulation toward mid-thickness. The effects of vapor barrier diffusion resistance, insulation thermal resistance, and changes in outdoor conditions (relative humidity, temperature, and wind speed) are reported in tabulated form and illustrated through pressure–position and temperature–position profiles.

In cryogenic air liquefaction systems, a major share of the mechanical energy consumption is associated with exergy destruction caused by heat transfer in recuperative heat exchangers. This study investigated the exergetic optimization of cryogenic gas separation systems by focusing on the Claude–Heylandt cycle as an advanced structural modification of the classical Linde–Hampson scheme. An exergy-based analysis demonstrates that minimizing mechanical energy consumption requires a progressive reduction in the temperature difference between the hot forward stream and the cold returning stream toward the cold end of the heat exchanger. This condition was achieved by extracting a fraction of the high-pressure stream and expanding it in a parallel expander, thereby creating a controlled imbalance in the heat capacities between the two streams. The proposed configuration reduces the share of exergy destruction associated with heat transfer in the recuperative heat exchanger from 14% to 3.5%, leading to a fourfold increase in the exergetic efficiency, together with a 3.6-fold increase in the liquefied air fraction compared with the Linde–Hampson cycle operating under identical conditions. The effects of key decision parameters, including the compression pressure, imposed temperature differences, and expander inlet temperature, were systematically analyzed. The study was further extended by integrating an air separation column into the Claude–Heylandt cycle and optimizing its configuration based on entropy generation minimization. The optimal liquid-air feeding height and threshold number of rectification trays were identified, beyond which further structural complexity yielded no thermodynamic benefit. The results highlight the effectiveness of exergy-based optimization as a unified design criterion for both cryogenic liquefaction and separation processes.

Air liquefaction systems are essential in cryogenic engineering and energy storage, yet their performance is often constrained by significant exergy destruction. This study develops an exergy-based assessment of the Linde–Hampson air liquefaction cycle to identify dominant sources of inefficiency and explore strategies for improvement. The analysis shows that throttling (≈41%) and compression (≈40%) represent the major contributions to exergy losses, followed by finite-temperature heat transfer (≈15%) in the recuperative heat exchanger. To mitigate these losses, fractional throttling and optimized inlet conditions are proposed, leading to reduced compressor work and improved overall efficiency. A comparative study of a two-stage throttling configuration demonstrates a decrease in throttling-related exergy destruction to approximately 30%. Reverse Pinch analysis is employed to verify the thermal coupling of hot and cold streams and to determine the minimum feasible temperature difference. The design optimization of the recuperative heat exchanger identifies an optimal velocity ratio that minimizes pressure losses and quantifies how compression pressure affects the required heat transfer surface area. The results provide a systematic framework for improving the thermodynamic performance of air liquefaction cycles, highlighting exergy analysis as a powerful tool for guiding structural modifications and functional optimization.

As the world’s population grows and energy demand increases, there is a need to switch from fossil fuels to renewable energy. In order to preserve the environment and meet these growing demands, especially for cooling applications, trigeneration systems could be the answer. The aim of this work is to provide a structured overview of the current state of the art in the field of trigeneration (CCHP) systems. Firstly, these systems and their applications are presented. An overview of the different indicators used to describe the performance of these systems is given. A comparison between CCHP (combined cooling, heat, and power) systems is made. Finally, improvements and other concepts found in the literature are presented. This review will later serve as a basis for the exergo-economic optimization of a low-temperature CCHP system based on renewable energy sources. Therefore, more attention is given to the technologies used for such systems.

This study focuses on the compression area of a cryogenic air separation unit (ASU). The mechanism of exergy consumption in the compressor was revealed. The influence of the compression ratio and of the isentropic efficiency per stage give arguments for proper choice of these decisional parameters. For the purchase cost of the compressor, an exergoeconomic correlation based on the exergetic product represented by the compression ratio and the isentropic efficiency as the Second Law coefficient of performance was used instead of the common thermo-economic one based only on the cost of materials. The impact of the suction temperature on the compressor operating performance is shown, making the gap between the compression stage and the associated intercooler. After optimization of the global system, a specific exergy destruction is assigned to each inter-stage compression cooler. To fit this optimum exergy consumption, a design procedure for the inter-stages and final coolers based on the number of heat transfer units (NTU-ε) method and the number of exergy units destroyed (NEUD) is shown. Graphs are provided that make the application of the method straightforward and much easier to use compared to the usual logarithmic mean temperature difference. A 25% increase in the compression ratio per stage leads to a decrease in the exergy efficiency of 3%, while the purchase cost of the compressor rises by 80%. An increase in the isentropic efficiency of the compressor from 0.7 to 0.85 leads to an increase in the exergetic performance coefficient of 21%, while the compressor purchase cost triples.

To improve the efficiency of a diesel internal combustion engine (ICE), the waste heat carried out by the combustion gases can be recovered with an organic Rankine cycle (ORC) that further drives a vapor compression refrigeration cycle (VCRC). This work offers an exergoeconomic optimization methodology of the VCRC-ORC group. The exergetic analysis highlights the changes that can be made to the system structure to reduce the exergy destruction associated with internal irreversibilities. Thus, the preheating of the ORC fluid with the help of an internal heat exchanger leads to a decrease in the share of exergy destruction in the ORC boiler by 4.19% and, finally, to an increase in the global exergetic yield by 2.03% and, implicitly, in the COP of the ORC-VCRC installation. Exergoeconomic correlations are built for each individual piece of equipment. The mathematical model for calculating the monetary costs for each flow of substance and energy in the system is presented. Following the evolution of the exergoeconomic performance parameters, the optimization strategy is developed to reduce the exergy consumption in the system by choosing larger or higher-performance equipment. When reducing the temperature differences in the system heat exchangers (ORC boiler, condenser, and VCRC evaporator), the unitary cost of the refrigeration drops by 44%. The increase in the isentropic efficiency of the ORC expander and VCRC compressor further reduces the unitary cost of refrigeration by another 15%. Following the optimization procedure, the cost of the cooling unit drops by half. The cost of diesel fuel has a major influence on the unit cost of cooling. A doubling of the cost of diesel fuel leads to an 80% increase in the cost of the cold unit. The original merit of the work is to present a detailed and comprehensive model of optimization based on exergoeconomic principles that can serve as an example for any thermal system optimization.

The study is looking for the optimal configuration of an air-to-water heat pump capable of heating water in a flow-return system. Carbon dioxide is used as a working agent. In the optimization strategy, exergy analysis is considered. Using exergy analysis, the magnitude and the location of any functional or constructive malfunction can be revealed. First, a standard one-stage heat pump system is considered. Due to the high exergy destruction in the throttling valve, the efficiency of the standard system is improved by coupling it with an auxiliary one. The coupling process is undertaken in an internal subcooler-evaporator-heat exchanger. By subcooling the CO2 before entering the throttling valve of the standard cycle, the exergy destruction associated with this process diminishes. To increase the efficiency of the globally coupled system, the heat transferred outside in the gas cooler of the auxiliary heat pump is used in the water heating process. The energetic and exergetic efficiencies of the coupled system increased by 19 % and 18.3 %, respectively, compared to the standard heat pump cycle.